Squirmy Science: Which Soil Types Do Earthworms Like Best?

Introduction
Have you ever dug a hole in the soil and noticed earthworms wriggling out of your way? Not only are earthworms good for birds and fish to eat, but these little animals actually work hard to help put food on your table, too. By adding nutrients to the soil they live in, earthworms are continually working to keep soil healthy so that most plants can grow well in it. To do their important work, earthworms prefer certain types of soils over others.

Background
Here's a riddle for you: What has five hearts, no eyes and helps feed all the people on Earth? The earthworm! This amazing animal is important for keeping soil rich in nutrients. Just as you need vitamins to stay healthy, such as the vitamin C you get from fruits and vegetables, plants need nutrients to grow properly as well—and they get them from the soil. The earthworm is able to transform dead plants and leaves (also called plant matter) into nutrients for the soil. Earthworms can make food for living plants from dead ones. They leave this food (the nutrients) behind in the soil where growing plants can absorb it.

In less than a day an earthworm can eat its entire weight in dead plant matter. So, for example, if you were an earthworm and weighed 50 kilograms, you'd need to eat about 50 kilograms of food every day! During this moving feast, an earthworm is also loosening and aerating (letting air get into) the soil to make it easier for many beneficial plants to grow in it.

Materials
• A cool area that is not brightly lit
• Measuring cups
• Dry sand (1.5 cups)
• Dry gravel (1.5 cups)
• Dead leaves, crumbled (1.5 cups)
• Dry potting soil (1.5 cups)
• Mixing bowl
• Spray bottle
• Water
• Spoon or fork
• Empty plastic box with a sealable lid, larger than a shoe box, but smaller than a laundry basket
• Earthworms (at least five); available at a bait shop; or use a small hand trowel or shovel to dig up earthworms from the ground (usually in shady, moist soil); or find them on the ground after it has rained
• A small container with a lid in which to hold the earthworms; put some damp soil and dead leaves inside the container for the worms, and keep them in a cool place until you are ready to do the activity

Preparation
• You can test this activity indoors or outside. Choose a location that is cool and in the shade (if you are testing outside) or in a dimly lit room (if you are testing indoors). If the plastic box is transparent, use an especially dark location, such as a closet.
• Put 1.5 cups of sand into the mixing bowl. Spray the sand with water (counting the squirts as you go) until it all feels wet, stopping before puddles of water appear; mix the moistened sand with a spoon or fork.
• Transfer the wet sand from the mixing bowl to one corner of the plastic box. Make sure the sand does not spill out over one quarter of the area of the bottom of the box.
• Rinse out and dry the mixing bowl. Spray the other "soils" (gravel, dried leaves and potting soil) in the mixing bowl (each separately) just as you did the sand. Use the same number of sprays of water for each soil type as you did for the sand. When each soil has been sprayed, transfer it in its own corner of the box. How wet does the sand feel compared with the gravel, dried leaves and potting soil?
• As you fill the four corners of the plastic box with the different "soils," make sure that none spill over and touch the other soils. Space the soils so that there is roughly two to five centimeters of space between the samples.

Procedure
• Place your plastic box in the cool, unlit area you chose. Check to make sure the soils are still separated and not touching one another.
• Place your five (or more) earthworms in the center of the box, in the middle of the four soil samples. Watch the earthworms for a minute or two. What do the earthworms do at first? How do they move? Do they follow one another? Did they all go into one soil or did they burrow into different soils?
• After watching them for a minute or two, close the lid on the box. Note what time it is. Let the earthworms stay undisturbed in the box for 24 hours.
• After 24 hours have passed, open the box. Do you see any worms outside of the soil?
• Carefully go through each pile of soil to look for the earthworms, moving the soil to another container or an empty part of the box. You may need to look very closely through the different soils to make sure you do not miss a small earthworm. Which soils have the most earthworms in them? Which soils have the least? Why do you think some of the soils hold more earthworms than others?
• Extra:Do earthworms stay in the soil in which they are put? Do this activity again but put one earthworm in each soil type. After 24 hours, is there still one earthworm in each sample, or are there more earthworms in one soil compared with another?
• Extra: Earthworms are blind, but they have special cells that can sense light. What kinds of light do earthworms avoid and what kinds do they not mind? Set up a box with damp potting soil in each corner. Cut out small holes in the box's lid at three corners. Put a small red-light flashlight pointing straight down above one corner, a small blue-light flashlight above another corner, and a small white-light flashlight above the third corner. Put an earthworm in the center of the box, put the lid on, and turn on all three flashlights. Which flashlight (or flashlights) does the earthworm go toward and which does it avoid? Did the earthworm try to stay away from all the lights or just certain colors? Do multiple earthworms behave the same way?

Observations and results
Were most of the earthworms in the leaves? Were some in the sand and potting soil? Were none in the gravel?

Earthworms eat dead plant matter, such as fallen leaves, and transform it into a nutritious substance that plants can absorb more readily. So, if they want a tasty meal, they would most likely be in the leaves. However, if there is organic matter (dead plant matter or animal matter, which they also eat) in the other soil types tested, the earthworms might be attracted to those soils as well. But just because an earthworm is in one of the soils does not mean that it is its ideal environment or that it wants to eat that soil. For example, you may find one or more earthworms in the gravel, but this could be due to factors other than the earthworm searching for a food source—the gravel's humidity, for example, might be more appealing or there may be possible chemoattractants on the gravel.

Besides adding nutrients to the soil, earthworms also keep the soil healthy by tunneling. As they eat, earthworms create tunnels that keep the soil open and full of paths for water and air. Without earthworms, the soil would be smashed down, or compacted, making it difficult for water and air to reach the roots of plants. Overall, it would be very hard to grow all the beautiful plants that give us oxygen and food without these wiggly, little creatures.

Cleanup
When you're done with this activity, gently and carefully return the earthworms to some soil outside.